This invention relates to a distortion compensation amplifier that compensates for distortion arising in an amplifier when the amplifier amplifies a signal. This invention particularly relates to a distortion compensation amplifier that, by reducing the distortion suppression required in a distortion removal loop, enables amplification with distortion compensation of signals over a broader band of frequencies than heretofore.
The transmitter unit of a base station used in a mobile phone system, Personal Handy phone System (PHS) or other such mobile telecommunications system uses an amplifier to amplify signals to be wirelessly transmitted. Since distortion occurs during the amplification of the signals by the amplifier, the transmitter unit is equipped with a circuit for compensating for the distortion.
FIG. 5 shows an example of a conventional distortion compensation amplifier unit. Specifically, it shows the configuration of a common amplifier capable of performing distortion compensation by the feedforward method. (A common amplifier uses an amplifier to amplify signals that include multiple frequency components.)
The illustrated common amplifier unit is equipped with three directional combiners 1, 5 and 9, two vector adjusters 2 and 7, a main amplifier 3, two delay lines 4 and 6, an auxiliary amplifier 8, a pilot signal generator 10, a receiver 11 and a control circuit 12. Also shown are an input terminal C and an output terminal D of the common amplifier unit.
The directional combiner 1, directional combiner 5 and the group of components between them (the vector adjuster 2, main amplifier 3 and delay line 4) constitute a circuit that functions as a distortion detection loop.
A multifrequency signal input to the distortion detection loop through the input terminal C is divided by the directional combiner 1. One divided signal is adjusted in amplitude and phase by the vector adjuster 2, amplified to a desired power by the main amplifier 3, and input to the directional combiner 5. Another divided signal is delayed by the delay line 4 and input to the directional combiner 5. The directional combiner 5 outputs (part of) the amplified signal received from the main amplifier 3 to the delay line 6, adds the (remaining part of) the amplified signal and the delayed input multifrequency signal received from the delay line 4 to detect the distortion component generated in the (remaining part) of the amplified signal in the main amplifier 3, and outputs the result to the vector adjuster 7.
The distortion occurring in the main amplifier 3 when the input multifrequency signal is amplified is, for example, intermodulation distortion.
Preferably, the distortion detection loop detects and outputs to the vector adjuster 7 only the distortion component produced in the main amplifier 3. Insofar as effective distortion compensation can be practically realized, however, the distortion component detected by the distortion detection loop can include other components such as the input multifrequency signal.
The amplified signal sent from the main amplifier to the directional combiner 5 contains the multifrequency signal component amplified by the main amplifier 3 and the distortion component generated in the main amplifier 3. On the other hand, the multifrequency signal sent from the delay line 4 to the directional combiner 5 does not contain the distortion component. By appropriately setting the amount of signal amplitude and phase adjustment by the vector adjuster 2 and the amount of signal delay by the delay line 4, therefore, the distortion component can be detected in the directional combiner 5 by subjecting the two input signals to reverse-phase addition with respect to the input multifrequency signals. In the reverse-phase addition, the two signals are, for example, added (synthesized) at the same delay, same amplitude and opposite phase (phase difference of 180 degrees).
The delay line 6, vector adjuster 7, auxiliary amplifier 8 and directional combiner 9 constitute a distortion removal loop downstream of the directional combiner 5.
In the distortion removal loop, the amplified signal output from the directional combiner 5 is delayed by the delay line 6 and input to the directional combiner 9. The distortion component detected by the directional combiner 5 is adjusted in amplitude and phase by the vector adjuster 7, amplified to a desired power by the auxiliary amplifier 8, and input to the directional combiner 9. The directional combiner 9 removes the distortion component from the amplified signal by adding the delayed amplified signal received from the delay line 6 and the distortion component received from the auxiliary amplifier 8 and outputs the result from the output terminal D.
The amplified signal sent from the delay line 6 to the directional combiner 9 contains the multifrequency signal component amplified by the main amplifier 3 and the distortion component generated in the main amplifier 3. On the other hand, the singal sent from the auxiliary amplifier 8 to the directional combiner 9 contains the distortion component. By appropriately setting the amount of amplitude and phase adjustment of the distortion component by the vector adjuster 7 and the signal delay by the delay line 6, therefore, the distortion component can be removed from the amplified signal in the directional combiner 9 by subjecting the two input signals to reverse-phase addition with respect to the distortion component.
The pilot signal generator 10 generates and outputs a prescribed signal as a pilot signal. The pilot signal is synthesized with the input multifrequency signal transmitted from the vector adjuster 2 to the main amplifier 3. The receiver 11 receives part of the signal output by the directional combiner 9 and detects the pilot signal contained in the received part of the signal. The vector adjuster 7 is controlled to diminish the pilot signal detected by the receiver 11. The distortion removal loop is thus optimized by using the pilot signal. The control circuit 12 controls the vector adjuster 2 based on the result of pilot signal detection by the receiver 11. The two vector adjusters 2 and 7 are provided to set optimum adjustment values in the respective loops.
The common amplifier unit shown in FIG. 5 deals not only with the input multifrequency signal and the distortion component it causes to be generated in the main amplifier 3 but also with the pilot signal and the distortion component it causes to be generated in the main amplifier 3. In the interest of simplicity, however, the ensuing explanation and the associated drawings will touch on only the input multifrequency signal and the distortion component generated as a result thereof and no explanation or elements in the drawings will be presented regarding the pilot signal and the distortion component generated as a result thereof.
FIG. 6 is a graph showing an example of the amplified signal output by the main amplifier 3 when a two-component signal composed of signal component of a frequency of f0 [MHz] and a signal component of a frequency f1 [MHz] is input to the input terminal C of the common amplifier unit. The horizontal axis of the graph represents frequency [MHz] and the vertical axis represents signal level. It can be seen from this graph that when the main amplifier 3 amplifies the input multifrequency signal composed of the signal components of frequencies f0 and f1, intermodulation distortion occurs that consists of low-frequency side distortion at a frequency of {f0xe2x88x92(f1xe2x88x92f0)} [MHz] and high-frequency distortion at a frequency of {f1+(f1xe2x88x92f0)} [MHz]. The interval between two frequencies at which these distortions occur increases in proportion as the bandwidth of the signal input through the input terminal C increases.
FIG. 7 is a graph showing an example of the amount of distortion suppression required by the amplified signal after distortion removal output from the output terminal D of the common amplifier unit. The horizontal axis of the graph represents frequency [MHz] and the vertical axis represents signal level.
FIG. 8 is a graph showing an example of the distortion suppression characteristic Q2 of the distortion removal loop required with respect to an input multifrequency signal composed of f0 and f1 frequency components. The horizontal axis of the graph represents frequency [MHz] and the vertical axis represents distortion suppression [dB].
In distortion compensation using the feedforward method, the practice is, for instance, to extract the distortion component in the distortion detection loop and finally remove the distortion by synthesizing the amplified distortion component and the distortion component contained in the signal amplified by the main amplifier 3 at the same delay, same amplitude and opposite phase. The distortion suppression characteristic of the distortion removal loop therefore requires distortion suppression in a 3xc3x97(f1xe2x88x92f0) [MHz] band.
In this case, assuming that, as shown in FIG. 6, the level of the distortion generated in the main amplifier 3 is X [dB] smaller than the level of the amplified input multifrequency signal and that, as shown in FIG. 7, the level of the distortion required at the output terminal D of the common amplifier unit is Y [dB] smaller than the level of the amplified input multifrequency signal, it follows that the amount of distortion suppression of the distortion removal loop required at the distortion frequencies {f0xe2x88x92(f1xe2x88x92f0)} and {f1+(f1xe2x88x92f0)} is Yxe2x88x92X=Z [dB], where Y greater than X.
FIG. 9(a) is a graph showing an example of how the amount of distortion suppression of the distortion removal loop realized differs among different sets of amplitude deviation d value [dB] and phase deviation xcex8 value [deg] when component between the delay and AMP paths. The horizontal axis of the graph represents amplitude deviation d value [dB] and the vertical axis represents phase deviation xcex8 value [deg]. As termed here, xe2x80x9cdelay line pathxe2x80x9d refers to the signal path equipped with the delay line 6 and xe2x80x9cAMP pathxe2x80x9d to the signal line path equipped with the vector adjuster 7 and the auxiliary amplifier 8.
As a simplified model of the distortion removal loop, FIG. 9(b) shows a circuit composed of a distributor 31 for dividing the input signal into signals of the same voltage V0, a vector adjuster 32 for adjusting the amplitude and phase of one divided signal, and a directional combiner 33 for adding the adjusted one divided signal and another divided signal.
In the example shown in FIG. 9(a), when the amplitude deviation d value is within xc2x10.1 [dB] and the phase deviation xcex8 value is within xc2x11.0 [deg], the distortion suppression of the distortion removal loop is 35 [dB]. As shown by the broken lines in FIG. 7, this distortion suppression corresponds to the level that the distortion component generated in the main amplifier 3 should be suppressed and, as shown in FIG. 8, corresponds to the distortion suppression required at the distortion component frequencies of {f0xe2x88x92(f1xe2x88x92f0)} and {f1+(f1xe2x88x92f0)}.
When a conventional distortion compensation amplifier like the one shown in FIG. 5 is input with a signal composed of two frequency components whose frequencies are at opposite ends of the band used, a broadband property enabling sufficiently accurate distortion removal in a frequency band three times that of the used band is required. However, the technology of the prior art can achieve only a limited broadband property. This disadvantage has made it difficult to realize a broadband feedforward type amplifier unit.
The present invention was accomplished to overcome this drawback of the prior art and has as an object to provide a distortion compensation amplifier that by reducing the distortion suppression required in the distortion removal loop enables amplification with distortion compensation of signals of broader band than heretofore
The present invention achieves these objects by providing a distortion compensation amplifier comprising a distortion detection loop and a distortion removal loop. The distortion detection loop divides the signal (the signal to be amplified) into two signals, amplifies one divided signal with an amplifier and uses the amplified signal and another divided signal to detect a distortion component generated and introduced into the amplified signal in the amplifier. The distortion removal loop effects distortion compensation by removing the detected distortion component from the amplified signal produced by the amplifier.
More specifically, the distortion detection loop includes filter means for reducing the distortion component of the amplified signal output by the amplifier that was generated and introduced into the amplified signal in the amplifier and detects the distortion component reduced by the filter means. The distortion removal loop removes the detected distortion component from the amplified signal that was produced by the amplifier and whose distortion component was reduced by the filter means.
Owing to the reduction of the distortion component detected by the distortion detection loop, the distortion component to be dealt with by the distortion removal loop is reduced. The amount of distortion suppression required in the distortion removal loop is therefore less than heretofore. This enables amplification with distortion compensation of signals over a broader band than heretofore. Specifically, it becomes possible, for example, to realize a broadband feedforward type amplifier unit, which has not been readily possible to implement using conventional technologies. The distortion removal loop deals with the distortion component detected by the distortion detection loop and the distortion component contained in the amplified signal input from the distortion detection loop.
The signal to be amplified can be any of various types but the present invention is particularly suitable for amplification of broadband signals.
The amplifier can be any of various types and can be a single amplifier or multiple amplifiers. Use of a common amplifier is particularly suitable in the present invention.
The filter means can be any of various types, such as a band-pass filter, low-pass filter or high-pass filter. Two or more filters may be used in combination. When the signal to be amplified includes two frequency components and compensation is carried out with respect to paired distortions occurring at frequencies symmetrically located on the low-frequency and high-frequency sides, a preferable embodiment can be configured by using a band-pass filter that reduces (attenuates) both distortions of the pair by the same amount of reduction (attenuation).
The degree to which the filter means reduces the distortion component of the amplified signal output by the amplifier that was generated and introduced into the amplified signal in the amplifier (the degree to which the distortion removal loop reduces the distortion) can be any of various degrees.